用于空间引力波探测的高精度皮瓦级弱光锁相控制研究

In the space gravitational waves (GWs) detection, one challenge is that the received light is as weak as pico-Watt level because of the laser diffraction between the two spacecraft with the distance of about millions of kilometers. Hence, an optical phase-locked-loop must be equipped to transfer the phase information of the incoming light back to the interferometer on the remote spacecraft. The current researches show that pico-Watt weak light phase locked noise is dominated by the shot noise (~100μrad/√Hz). However, the analog phase locked loop can’t meet the requirement of auto-locking in the space application. Moreover, the long-term phase locking is absolutely important for GWs detection and requires far more experimental research. In this project, we will research on the non-linear phase locked loop in the condition of low signal-to-noise ratio (SNR). Based on the theoretical analysis and simulation, the digital phase locked loop shows the main noises, which will be depressed in our experiment. On the basis of the research on the high precision digital phasemeter and digital frequency and phase detector, we will develop the key technology units such as the pico-Watt photo-detector, the auto-gain controller, and the low noise feedback driver. The main innovation is that we will use advanced control algorithm for auto-locking and re-locking. We will finally realize the high precision digital phase locked loop with the out-loop phase noise of 20μrad/√Hz. In the condition of the weak light with the power of 100 pico-Watt, the out-loop phase noise will be limited by the shot noise and the loop be locked continuously more than one month. The whole research will improve the key technique of weak light digital phase locked loop, and the experimental results of high precision, auto-locking and long-term locking can meet the demands of the space GWs detection.

在空间引力波探测中，卫星间距达到百万公里，接收光功率衰减至百皮瓦量级，必须采用光学外差锁相实现主从激光的相干接收和放大。国内外的研究表明，皮瓦量级的弱光锁相噪声受到散粒噪声的限制（~100μrad/√Hz）。但是，所采用的模拟锁相控制系统无法满足空间应用的自动锁相控制需求，且缺乏弱光锁相长期锁定的实验研究。本项目将探索弱信号锁相的非线性理论，结合仿真分析弱光锁相的噪声来源，并提出相应的噪声抑制方法。在高精度数字相位计、数字鉴频鉴相器的基础上，研制皮瓦级弱光传感、自动增益控制、低噪声反馈驱动等关键技术单元。主要创新之处是研制基于高级控制算法的自动锁定与重锁技术，最终实现高精度的数字锁相控制系统，环外相位噪声达到20μrad/√Hz；在弱光功率为100pW的条件下，连续锁相时间超过一个月，且锁相环外相位噪声达到散粒噪声限制，从而满足空间引力波探测中弱光锁相的高精度、自动和长期锁定的需求。

在空间引力波探测中，卫星间距达到百万公里，接收光功率仅为百皮瓦量级，必须采用光学外差锁相实现主从激光的相干接收和放大。国内外的研究表明，皮瓦量级的弱光锁相噪声受到散粒噪声的限制。但是，所采用的模拟锁相控制系统无法满足空间应用的自动锁相控制需求，且缺乏弱光锁相长期锁定的实验研究，低频段的残余相位噪声难以达到空间引力探测的分辨率要求。针对项目研究目标，完成了皮瓦量级弱光锁相控制系统的误差分析、建模仿真；完成了相位测量电路的建模和噪声评估；研制出了低噪声高带宽的皮瓦级弱光传感电路、弱信号增益放大电路、低噪声反馈驱动电路、以及基于FPGA的全数字锁相控制电路，环外相位噪声达到20μrad/√Hz（3mHz~1Hz）。针对弱光锁相长期锁定的问题，通过弱信号频率检测、频率扫描、数字控制算法等技术途径实现了自动锁定与重锁的功能。针对弱光锁相低频噪声抑制的问题，基于双频声光衍射实验方案，搭建了激光干涉光路，实现了fW-pW量级的弱光干涉信号的相位检测及pW级的弱光锁相控制，完成了弱光功率的测试和标定，以及低频噪声分析和实验测试。在弱光功率100pW的条件下，相位噪声本底在10mHz以上频段达到了弱光散粒噪声的限制，约为100μrad/√Hz，达到了本项目的研究目标。该项目研究为未来星间应答式干涉仪、激光卫星编队飞行以及空间引力波探测等提供了技术支撑。